In the practice of medicine it is often desirable to introduce an intravenous fluid into a patient. The rate of introduction of the fluid to the patient is dependent upon a number of different factors including the weight, age, sex, physical state of the patient, and nature of the fluid to be introduced. Thus, in systems adapted for intravenous fluid introduction the rate of fluid introduction to the patient should be adjustable and measurable to assure that the patient receives an optimum benefit from the fluid. A very desirable feature of any such system is the automatic sensing of flow faults, i.e., situations where fluid is not being properly administered to the patient.
In copending application Ser. No. 258,361, filed concurrently by Thomas C. Thompson for an "Intravenous Drug Additive Delivery System with Electronic Control" and assigned of record to the assignee of record of this application, a system is disclosed and claimed for providing the introduction of intravenous fluid on a precisely controlled gravitational basis to a patient. Disposable cassettes are used in connection with apparatus to control the flow rate of intravenous fluid. The replaceable cassette has a cassette input line and a cassette output line which are connected to a source of fluid and the patient respectively. The cassette is manufactured of transparent plastic having various passageways and a membrane chamber molded therein. A flexible diaphragm membrane is disposed within the chamber to define a right and left side of the chamber. As the volume of one of the sides of the chamber is increased by movement of the membrane, the volume of the other side is correspondingly decreased. Each side of the chamber has an inlet and outlet line. The right and left inlet lines communicate with the cassette input line, and the right and left outlet lines communicate with the cassette output line.
Both inlet lines and both outlet lines are controlled by independently operable valves. The valves are operated alternately to allow periodic flow of fluid to the patient. For example, the inlet line to the left-hand side of the chamber is opened and the outlet line of the right-hand side of the chamber is opened, while the other two valves are closed, such that fluid flows from the right side of the chamber into the patient and into the left side of the chamber from the fluid source. Alternately, the inlet line of the right side of the chamber and the outlet line of the left side of the chamber are opened, the other valves now being shut, such that fluid flows into the patient from the left side of the chamber and from the fluid source into the right side of the chamber.
The opening and closing of the four valves is controlled by a microprocessor to produce a given flow rate into the patient. In one embodiment the flow rate has a range of 1 to 300 milliliters per hour. In the preferred embodiment, the chamber has a volume of one-tenth of a milliliter. Therefore, at a flow rate of 300 milliliters per hour the membrane shifts from one side of the chamber to the other side every 1.2 seconds. At a flow rate of one milliliter per hour the membrane shifts once every six minutes. Thus, the system disclosed and claimed in application Ser. No. 258,361 is advantageous because the cassette meters fluid by alternately shifting a membrane in a chamber such that one side of the chamber is filled while the other side is emptying and vice versa. The flow rate is directly controlled by the timing of the opening and closing of the valves associated with the chamber. No provision is made for the sensing of flow faults.
A somewhat similar system for intravenous introduction of a fluid is disclosed in U.S. Pat. No. 4,204,538 to Cannon. In Cannon, a multi-part cassette forming a chamber divided by a flexible membrane is shown. Four valves are provided to control the opening and closing of the inlet and outlet lines for each side of the chamber. However, in Cannon the membrane is directly attached to a ferrous rod which moves within a coil to produce a signal indicative of the position of the membrane within the chamber. The opening and closing of the various valves is controlled by sensing the position of the membrane by the ferrous rod transducer. When the transducer indicates that the membrane has reached its furthest extension in one direction, the valves are switched into the alternate configuration to begin the movement of the membrane in the direction of the other furthest extension. The line to the patient is constricted by a clamp downstream from the cassette. The constriction of the clamp is variable to produce different flow rates. The periodic switching of the valves in response to a control intelligence plays no part in the setting of the flow rate in Cannon. Further, the physical attachment of the position-sensing transducer to the membrane severely limits the dynamic range of the membrane. No flow fault sensing is provided in Cannon.
In U.S. Pat. No. 4,207,871 to Jenkins, flow rate, as opposed to flow fault, sensing is provided in a membrane system. The ferrous rod transducer of Cannon is excited by an oscillator to produce a signal which varies with the position of the membrane. The signal is integrated to determine the actual flow rate of the system. This flow rate is compared to the desired flow rate to generate a feedback signal to be applied to the clamp downstream. The flow rate is controlled in this manner, but no provision is made for sensing a flow fault in the system. Like in the Cannon system, the transducer in the Jenkins system severely limits the dynamic range of the membrane.
In systems of the type described above, the proper functioning of the system is entirely dependent on the back and forth movement of the membrane. If the membrane cannot or will not move for some reason, such as a ruptured membrane or any valve not functioning properly, the system will cease delivery of the fluid to the patient. The quick identification and repair of such a flow fault is necessary for the proper treatment of the patient, and thus a need has arisen for a system to determine and indicate such flow faults.